Тепловые методы
УДК 620.179.13
THERMOGRAPHIC, ULTRASONIC AND OPTICAL METHODS: A NEW DIMENSION IN VENEERED WOOD DIAGNOSTICS1
S. Sfarraa*, P. Theodorakeasb**, N.P. Avdelidisb, and M. Kouib a Department of Mechanical, Management and Energy Engineering, Las. E.R.
Laboratory, University of L'Aquila, Piazzale E. Pontieri 1, Roio Poggio, I-67100, L'Aquila (AQ), Italy b Department of Materials Science & Engineering, School of Chemical
Engineering, NDT Lab, National Technical University of Athens, Iroon Polytechniou 9, Zografou Campus, 15780, Athens, Greece * E-mail: stefano.sfarra@univaq.it ** E-mail: pantheod@mail.ntua.gr
Abstract. Modern production techniques in the wood-based industry reached a high quality standard at high output rates. While the speed of the production machines increases, it is necessary to introduce modern and faster working online inspection methods to supervise constantly the material for defects. For example, thermographic cameras are able to detect not only invisible defects within wood-based materials like laminated particle and fiberboards, but can be used also to detect defects in lumber and veneered wood [1—4]. In the latter case, there is the need to inspect more accurately the final pieces, given the exponential growth in worldwide sales. Therefore, in order to minimize adhesion problems [5], detecting surface and sub-surface cracks, define the geometry of the sub-surface detachment, in the veneered wood products, an integrated non-destructive test method is needed both during the production process that after to it [6]. Our system can provide a continuous control of the process and the product. In fact, this study compares the performance of transient thermography, three optical methods and ultrasonic testing applied together on a veneered wood sample with real and fabricated defects. The use of phase-shifting holography correlated to Double-Exposure HI and the wavelet transform applied as fusion of images between Thermographic Signal Reconstruction and Double-Exposure HI, are explored in this work.
Key words: transient thermography, holographic interferometry, digital speckle photography, digital speckle shearography, ultrasonic testing, defect, wood.
ТЕРМОГРАФИЧЕСКИЙ, УЛЬТРАЗВУКОВОЙ И ОПТИЧЕСКИЙ МЕТОДЫ: НОВОЕ ИЗМЕРЕНИЕ В ДИАГНОСТИКЕ СЛОИСТОЙ ДРЕВЕСИНЫ
С. Сфарра, П. Теодоракис, Н.П. Авделидис, М. Коуи Факультет механики, управления и энергетики, Университет г. Аквилла, Италия Факультет материаловедения и машиностроения, Школа химического машиностроения, Лаборатория неразрушающего контроля, Технический университет г. Афины, Греция
Современные технологии производства в лесной промышленности достигли высоких стандартов качества при высокой производительности. Рост скорости производства приводит к необходимости применения быстрых онлайновых методов контроля. Например, термографические камеры способны обнаруживать не только невидимые глазом дефекты древесных материалов, таких как ламинированные и древесно-волокнистые плиты, но они могут быть использованы также для обнаружения дефектов покрытой шпоном древесины [1—4]. В последнем случае необходимость контроля готовой продукции особенно велика в связи с экспоненциальным ростом продаж по всему миру. Чтобы свести к минимуму проблемы с адгезией [5], с целью выявления поверхностных и подповерхностных трещин и определения отслоений в фанерованных изделиях из древесины, необходимо
1 The article is published in the original.
применение интегрированного неразрушающего метода испытаний продукции как в процессе, так и на конечном этапе производства [6]. Наша система может обеспечить непрерывный контроль процесса и продукта. Настоящее исследование позволяет оценить эффективность скоростной термографии, трех оптических методов и у. з. контроля, примененных комплексно на шпонированных образцах древесины с реальными и искусственными дефектами. Рассмотрено использование фазосмещенной голографии совместно с методом двойной экспозиции, а вейвлет-преобразование применено для слияния изображений, полученных реконструкцией термографического сигнала и двойной экспозицией.
Ключевые слова: древесина, фанера, расслоение, термография, ультразвук.
INTRODUCTION
Engineered wood, also called composite wood, man-made wood or manufactured board; includes a range of derivative wood products which are manufactured by binding the strands, particles, fibers, or veneers of wood, together with adhesives, to form composite materials. Fig. 1 shows a sketch of the section of a typical veneered wood piece. These products are engineered to precise design specifications which are tested to meet national or international standards. In woodworking, veneer refers to thin slices of wood, usually thinner than 3 mm, that typically are glued onto core panels (typically, wood, particle board or medium-density fiberboard) to produce flat panels.
Fig. 1. A schematic cross-section of a veneered wood panel.
Veneer is obtained either by "peeling" the trunk of a tree or by slicing large rectangular blocks of wood known as flitches. The appearance of the grain and figure in wood comes from slicing through the growth rings of a tree and depends upon the angle at which the wood is sliced. Each slicing processes gives a very distinctive type of grain, depending upon the tree species. In any of the veneer-slicing methods, when the veneer is sliced, a distortion of the grain occurs. As it hits the wood, the knife blade creates a "loose" side where the cells have been opened up by the blade, and a "tight" side. Veneering is an ancient art, dating back to the ancient Egyptians who used veneers on their furniture and sarcophagi.
Diagnostic investigations of wood have the following objectives: I) accurate determination of its properties; II) recording of the degree and extent of defects and damages; III) characterization of the nature and intensity of damage; IV) determination of the cause of defects and damages; V) monitoring remedial treatments. Test methods may be classified as destructive or nondestructive, recognizing that there may be destructive methods which inflict only minor damage and might thus be termed near-nondestructive. Both destructive and non-destructive methods are used for wood in scientific research and industrial quality control, but tests based on the destructive removal of tests specimens from art objects and other cultural property can be acceptable only in rare, exceptional cases. For cultural property, the determination of individual wood properties is of much less importance than the determination of the condition both before and after conservation and restoration treatments. Detection and evaluation of the condition of wood are also of great interest in forestry and the wood industry. In forestry, this would mean the early detection of internal decay and of bark or wood borers
before they can cause catastrophic damage. In the wood industry, attention is paid to locating defects in logs or timbers, but also finding discolorations, decay, and insect damage. Whether it be forestry, the wood industry, or cultural property, a common goal is to use methods which are as close as possible to being nondestructive in order to prevent further damage. The condition analysis of logs and lumber can be made by mechanical [7], electrical [8], optical [9], acoustic [10], ultrasonic [11], thermographic [12, 13], radiographic [14], nuclear magnetic [15], chemical [16], and biological methods [17] or possibly a combination of several of these methods [18, 19].
15 cm
Fig. 2. Veneered wood sample (front of view) and paths of ultrasound measurements.
In this work, transient thermography, holographic interferometry, digital speckle photography, digital speckle shearography and ultrasonic testing techniques have been employed in order to detect fabricated and real defects at several depths in a veneered wood sample (Fig. 2).
TRANSIENT THERMOGRAPHY (TT) — THERMOGRAPHIC SIGNAL RECONSTRUCTION (TSR)
During infrared thermographic inspections, there are two approaches that can be used: passive [20] and active [21]. The passive approach is usually used in the investigation of materials that are at different (often higher) temperature than ambient, whereas in the case of the active approach an excitation source, such as optical flash lamps, heat lamps, hot or cold air guns, etc., is employed with the intention of inducing thermal contrasts [22]. In the active approach of the infrared thermographic NDT&E (non-destructive testing and evaluation), one of the widely used approaches is pulsed thermography. In this technique, also known as transient thermography, the surface under investigation is pulse heated (time period of heating varying from ms to s) by various heating sources (i. e. flash lamps) and the resulting thermal transient at the surface is monitored using an infrared camera. The duration of the heating pulse depends on the thermal and physical properties of the materials, as well as its thickness.
5 Дефектоскопия, < 4, 2013
The heat flow into the sample is altered in the presence of a subsurface defect or feature, creating temperature contrast at the surface that is recorded by an infrared system [23]. In the 1980 s, Vavilov and Taylor [24] discussed the principles of active IR thermographic NDT expressing the ability to provide quantitative information about hidden defects or features in a material. Following the early work [24], thermographic NDT&E has been adopted by numerous groups worldwide [25—27]. The capabilities of the technique for detecting and imaging subsurface defects have been greatly enhanced [28] and the defects imaging process is now well understood [29—31]. The aim of this work however, is to investigate the potential of transient thermography applied together with optical and ultrasonic testing techniques. Since transient thermography [32] is an inherently near surface technique whose effectiveness will decrease with skin thickness and as it is fairly d
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